1. Field of the Invention
The present invention relates to a SiC thin-film thermistor using a sputtered SiC film as a temperature sensitive resistor and, more particularly, to a SiC thinfilm thermistor of a type wherein no welding technique is used to connect lead wires.
2. Description of the Prior Art
Numerous temperature sensors comprising metals, metal oxides and other materials have long been used in various commercial products. Of them, a SiC thin-film thermistor is generally considered suitable as a temperature sensor for use in cooking products such as, for example, a microwave oven or the like, because it is known from any one of U.S. Pat. Nos. 4,359,372, 4,424,507 and 4,968,964 to have a capability of detecting a wide range of temperature ranging from 0.degree. to 500.degree. C. with a high sensitivity.
The known thermistor generally comprises a thermistor element and a package enclosing the thermistor element. The thermistor element comprises a sputtered SiC film formed on an electrically insulating substrate made of alumina. The package includes a pair of external lead wires fixed firmly to an insulating body. This type of known thermistor is generally regarded having a problem in that it requires a complicated structure and, also, a complicated manufacturing process which will now be discussed in detail.
The thermistor element is manufactured by the formation of a pair of thick-film electrodes on one surface of an alumina substrate by the use of a firing technique that is carried out before a sputtering. Each of those thick-film electrodes formed on the alumina substrate has first and second portions connected together: The first portion of the respective thick-film electrode is of a generally comb-like configuration and determinative of a thermistor resistance and the second portion thereof has a surface area of 0.25 to 1 mm.sup.2 so large as to permit an internal lead wire to be subsequently connected therewith by the use of a welding technique.
The SiC film is subsequently deposited on a surface of the alumina substrate by the use of a radio frequency sputtering technique. Since the internal lead wires cannot be welded to the SiC film, the second electrode portion of each thick-film electrode must be masked to avoid a deposition of the SiC film during the sputtering. Also, since the alumina substrate is kept at a temperature generally higher than 600.degree. C., the material used for masking the second electrode portion to avoid the deposition of the SiC film is required to have a property capable of withstanding the elevated temperature. An alumina plate of generally elongated shape corresponding to the slender shape of the second electrode portion is generally considered one of the most suitable materials for masking purposes because of its high-temperature withstanding capability.
However, in practice, a single sputtering cycle can produce a number of, for example, 1,000 and 1,500, thermistor elements and, therefore, it is a customary practice to arrange a corresponding number of alumina substrates regularly in two dimensions on a flat substrate holder during the sputtering. At the same time, the entire number of second electrode portions of the electrodes which will eventually be formed in those thermistor elements must be masked. While the use of the alumina plate is feasible as the masking material as discussed above, alumina is very hard, but brittle and it is not possible to prepare a single alumina plate of a size sufficient to cover the entire number of second electrode portions. In view of this, it is a conventional practice to manually place the individual alumina plates over the corresponding second electrode portions of the electrodes on the respective alumina substrates which are arranged regularly in two dimensions on the flat substrate holder.
The manual placement of the masking members over the individual electrode portions of the electrodes is not only complicated and cumbersome, but also time-consuming. Although the single masking member of a size sufficient to cover the total surface area of the electrode portions may readily be available if a metallic material other than alumina is used, such a single masking member tends to exhibit a large thermal deformation and can therefore not be used in high-temperature environments in excess of 600.degree. C.
In addition to those problems discussed above, the conventional manufacture of the SiC thin-film thermistor has an additional problem found after the formation of the thermistor element. Specifically, after the formation of the thermistor element, the internal lead wires are welded at one end to the respective second electrode portions of the respective electrodes. Each of the internal lead wires is generally employed in the form of a platinum wire of about 0.1 mm in diameter. Since the internal lead wires are physically weak, these internal lead wires welded at one end to the corresponding second electrode portions are in turn welded at the opposite end to external lead wires each having a practically acceptable physical strength and fixed to an insulating body. Each of those external lead wires is generally in the form of, for example, a generally elongated Fe-Cr alloy plate of 1 to 2 mm in width and 0.3 to 0.6 mm in thickness. In practice, those external lead wires are manually individually welded to the internal lead wires and, therefore, require a complicated and time-consuming procedure.